The present invention relates to a method to obtain contamination free laser mirrors and passivation using dry etching and deposition.
One of the key factors that dictates manufacturing of reliable 980 nm pump lasers is the quality of the laser facet. Passivation is a common technique in the semiconductor business. All semiconductors need a thin film as a barrier against impurities. Impurities will act as defects and change the electrical and optical behavior or impair the crystalline structure in general, by oxidization for example. For silicon chips the passivation is performed automatically by exposing the chip to oxygen in the atmosphere. The oxygen will form a protective SiO2-layer. The oxidization of GaAs based lasers is highly detrimental for the optical performance, therefore another materials to be applied on the laser facet.
Degradation of laser facets by light absorption is known to lead to sudden failure by catastrophic optical damage (COD) and has been one of the major causes for device failure. This is a serious concern especially for high power operation (usually beyond 150 mW). The onset of COD is attributed to light absorption at the output facet and subsequent non-radiative recombination via surface states. The light absorption and non-radiative recombination increases the temperature and that results in band-gap reduction. This process acts as a positive feedback until the facet temperatures become very high and COD occurs.
Therefore, to suppress this undesirable effect, at least one of the two main factors, light absorption and surface recombination, has to be minimized. The surface recombination is promoted by an increase in either surface-state density and/or number of impurities (traps) at the surface. The light absorption can also be minimized by a so-called window comprising a thin layer of an inactive material between the facet and the active layer lying behind the facet. In this case the bandgap of the window structure should be higher than the bandgap of the active layer. The minimization of these can be accomplished by suitable surface passivation coatings or treatments.
U.S. Pat. No. 4,448,633 discloses a method to passivate type III-V compound semiconductor surfaces by exposure to a low-pressure nitrogen plasma. The III element forms III element-nitride. This process is referred to as nitridation. The resultant articles have an III element-nitride surface layer, which protects the articles from environmental degradation while reducing the surface state density and permitting inversion of the surface layer. The nitridation is performed in two steps. The first occurs at low temperatures (400-500xc2x0 C.) to prevent decomposition of the surface by loss of V element. Exposure to nitrogen plasma with a pressure of 0.01-10 Torr results in an initial III-nitride layer having a thickness of about 20-100 xc3x85. The second step is performed at an elevated temperature (500-700xc2x0 C.) under the same plasma conditions. Here, the nitridation proceeds at a faster rate resulting in a thicker nitrided layer (200-1000 xc3x85). Under the present conditions, if the plasma pressure is in tie range 0.01 to about 0.5 Torr the resulting III-coating is polycrystalline, and is single-crystalline when the pressure is in the range 1 to 10 Torr.
U.S. Pat. No. 5,780,120 describes a method of preparing facets of lasers based on III-V compounds. The method comprises of the following operations:
1) The facets of the laser are cut.
2) The facets of the laser are placed in an enclosure in which there obtains a pressure of about 10-7 mbar to about 10-8 mbar, and they are subjected to a step of cleaning by irradiation with a pulsed laser.
3) The same pulsed laser is used to ablate a target so as to subject the exposed facets to a passivation operation, that is 2-20 xc3x85 of Si or GaN is deposited.
The deposition can be performed by pulsed laser ablation of a liquid gallium target in a nitrogen atmosphere with Electron Cyclotron resonance (ECR) plasma. Deposition of an additional film such as Diamond Like Carbon (DLC), silicon carbide SiC, or silicon nitride Si3N3, may be deposited using the same pulsed laser. These coatings are transparent at the wavelength of the laser and are resistant to oxidation. A cleaning step prior to the passivation stage may be performed in an atmosphere of chlorine or bromine, using a pulsed excimer laser. This document suggests that an additional coating is not necessary if GaN is deposited instead of Si. This also suggests that III-N layers are oxygen-proof.
U.S. Pat. No. 5,834,379 describes a process for synthesizing wide band gap materials, specifically GaN, employs plasma-assisted thermal nitridation with NH3 to convert GaAs to GaN. This method can be employed for forming layers of substantial thickness (on the order of 1 micron) of GaN on a GaAs substrate, Plasma-assisted nitridation using NH3 results in formation of predominantly cubic GaN. The objective of this document is to make sufficiently thick GaN layers and is not directly concerned with laser facet passivation. However, the basic principle relies on nitridation using a plasma source. Such approaches are being used in growth of GaN films.
The above patents address the concept of nitridation of III-V semiconductors using nitrogen plasma.
U.S. Pat. No. 4,331,737 describes an oxynitride film, which contains Ga and/or Al and has O/N ratio of at least 0.15. This film is obtained by relying on, for example, chemical vapour deposition (CVD) technique, The O/N ratio in the film may be varied by, for example, by varying the distance between the substrate and the substance-supply source, or by varying the proportion of an oxidising gas contained in a carrier gas. This film is used either as a surface passivation film of III-V compound semiconductors such as GaAs, or as an insulating film for active surface portions of IG-FET, or as an optical anti-reflective film.
EP0684671 describes a method, which comprises oxide reduction, hydrogen passivation and deposition of a protective coating layer. The method involves the same PECVD reactor for all steps to avoid oxygen exposure The cleaved facets (being exposed to air and thus oxidised) are loaded into the reactor. The first step uses hydrogen plasma, which both reduces the group V oxide content and passivates non-radiative recombination centres. The group III oxides are removed by ammonia plasma and the laser facets have their compositional stoichiometry condition restored and are free from contaminants. Coating is then done either by depositing SiN(x) or AlN(x). Minimum stress can also be obtained through creation of a compositional nitrogen gradient.
U.S. Pat. No. 5,668,049 discloses a method of making a GaAs-based semiconductor laser. A fully processed wafer is cleaved, typically in ambient atmosphere into laser bars. The laser bars are loaded into an evacuable deposition chamber (preferably an ECR CVD chamber) and exposed to H2S plasma. The hydrogen is believed to remove native oxides, while the sulfur bonds with Ga and As, thereby lowering the surface state density. Following the exposure, the cleavage facets are coated in the chamber with a protective dielectric (for example, silicon nitride) layer. The patent claims that this method can be practiced with high through-put, and can yield lasers capable of operation at high power.
U.S. Pat. No. 5,144,634 discloses a method for passivating mirrors in the process of fabricating semiconductor laser diodes. Key steps of the method are:
(1) providing a contamination-free mirror facet, followed by
(2) an in-situ application of a continuous, insulating (or low conductive) passivation layer.
This layer is formed with a material that acts as a diffusion barrier for impurities capable of reacting with the semiconductor but which does not itself react with the mirror surface. The contamination-free mirror surface is obtained by cleaving in a contamination-free environment, or by cleaving in air, followed by mirror etching, and subsequent mirror surface cleaning. The passivation layer consists of Si, Ge or Sb. A Si layer with a second layer containing Si3N4 is also claimed.
EP0474952 proposes another method of passivating etched mirror facets of semiconductor laser diodes for enhancing device reliability. The etched mirror facet is first subjected to a wet-etch process to substantially remove any native oxide as well as any surface layer which may have been mechanically damaged during the preceding mirror etch process. Then a passivation pre-treatment is applied whereby any residual oxygen is removed and a sub-mono-layer is formed which permanently reduces the non-radiative recombination of minority carriers at the mirror facet. As pre-treatment Na2S or (NH3)2S solutions can be used. The sulfur passivates the surface electronic states that otherwise are efficient non-recombination centers. Finally, the pre-treated mirror surface is coated with either Al2O3 or Si3N4 to avoid any environmental effect.
EP0774809 describes a method to offer a novel passivation layer that can result in improved reliability of semiconductor lasers having a laser cavity defined by laser facets. In a preferred embodiment, the passivation layer is a zinc selenide layer (e.g., 5 nm), formed on an essentially contamination-free laser facet. More generally, the passivation layer comprises at least one of Mg, Zn, Cd and Hg, and at least one of S, Se and Te. Typically, the facets are formed by cleaving in vacuum and followed by in-situ deposition of the novel passivation layer material on the facets.
U.S. Pat. No. 5,851,849 describes a process for passivating semiconductor laser structures with severe steps in the surface topography. The technique involves atomic layer deposition to produce the passivating layer which has exceptional coverage and uniformity, even in the case of trench features with trench aspect ratios as large as 5. In addition, the passivation produced by this process has excellent environmental stability, and affords protection against air born contaminant induced degradation. The coating process is carried out in a vacuum chamber. The primary feature of the process is the formation of the coating by a multiplicity of process cycles in which each cycle produces essentially an equivalent mono-layer of the passivating film. In the specific example described here the passivating film was Al2O3 and the reactant gases were trimethylaluminum [(CH3)3Al].
The above patents mainly address different passivation methods. Typically, the processes are complicated and involve at least two steps. In some cases, special techniques and/or materials (gases, precursors etc) are used. Never the less, most of these deal with means to reduce surface state density, which is one of the important factors to suppress COD.
The article xe2x80x9cCleaning of GaAs Surfaces with Low-Damage Effects Using Ion-Beam Millingxe2x80x9d by C. Lindstrxc3x6m and P. Tihanyi, the Journal IEEE Trans. on electron Devices, Vol.ED-30, NO.6, June 1983. With ion-beam milling of the laser diode mirror surface an etch depth of 50-100 {dot over (A)}ngstrxc3x6m reduces the oxygen atomic percentage by 97-99% as determined by Auger depth profiling. From the same report the difference between milling with heavy Ar ions and lighter N ions were demonstrated. The important result was that N ions had no measurable detrimental influence on the laser diode performance while milling with Ar ions affected the performance negatively in the milling process. After 140 {dot over (A)}ngstrxc3x6m milling depth with Ar ions the power output and power conversion efficiency started to decline. However, with the introduction of N ions in the milling process no parameter changes were observed for the milling depth studied i.e. 200 {dot over (A)}ngstrxc3x6m.
The effect of Ar ion milling followed by N ion milling on the laser performance is also described in this article. Here, the lighter N ions remove the damage caused by the heavier Ar ions and restore the deteriorated power output performance. The conclusion from these observations is that N ion milling smoothens the mirror facet to a uniform surface similar to what is observed for surfaces mechanically cleaved in the crystal plane with a correspondingly reduced number of surface states.
The Article xe2x80x9cLow resistance ohmic contacts an nitrogen ion bombarded InPxe2x80x9d, Ren et al, Appl. Phys. Lett. 65, 2165 (1994) reports on electrical and chemical properties of InP surfaces milled by low energy (100-300 eV) nitrogen-ions. Incorporation of nitrogen is evidenced by Secondary Ion Mass Spectroscopy (SIMS) analysis and formed poly-crystalline InN was identified by transmission electron microscopy (TEM). In the process, the native oxide on the sample surface is also removed by the milling.
The article xe2x80x9cNitridation of an InP (100) surface by nitrogen ion beamsxe2x80x9d, Suzuki et al, Appl. Surf. Sci. 162-163, 172 (2000) describes a study of nitridation of InP (100) by low energy nitrogen ion milling. The investigators used X-ray photoelectron spectroscopy (XPS) for chemical analysis and to identify the bonding states. The ion energy ranged from 100 eV to 1 KeV. The milled surfaces show Inxe2x80x94N, Inxe2x80x94Nxe2x80x94P and Pxe2x80x94N bonding states Disappearance of Inxe2x80x94Nxe2x80x94P upon annealing (400xc2x0 C.), suggests lower binding energies for these bonds compared to Inxe2x80x94N. However, nitridation efficiency decreases with increasing ion energy due to sputter erosion.
The article xe2x80x9cCharacterization of damage in InP dry etched using nitrogen containing chemistriesxe2x80x9d, C. F. Carlstrxc3x6m and S. Anand, submitted to J. Vac Sci. Technol. B (March 2001) addresses etching of InP using different of processes containing nitrogen in the etch-chemistry, including nitrogen ion milling. The surfaces are extremely smooth with rms roughness less than 1 nm with milling at 75 eV. A thin near surface nitrogen containing layer is present. A high temperature treatment (650xc2x0 C.) under phosphine, removes most of the incorporated nitrogen.
The article xe2x80x9cSynthesis of InNxP1-x thin films by N ion implantationxe2x80x9d, Yu et al, Appl. Phys. Lett. 78, 1077 (2001) describes implantation of nitrogen, which is carried out to form dilute InNxP1-x layers. Nitrogen ions were sequentially implanted with selected energies to form 350 nm thick layers and upon rapid thermal annealing (RTA) in flowing nitrogen (with proximity cap) the InNP alloy layers were formed.
Although, the articles above focus on different issues, the message is incorporation of nitrogen into InP during nitrogen ion milling. In addition, the results suggest that N binds to both In and P, the latter being less stable. The nitridation procedure needs to be optimized so as to have predominantly Inxe2x80x94N in the layer. At the same time the surface must be smooth. The last work listed (Yu et al.) above offers another means to form a nitrided layer, but it is restricted in that an all-InN layer is not obtained. But, it suggests that after nitridation by ion-milling, RTA may be an additional step that may be necessary.
Nitridation of GaAs has received a great deal of attention. One of the primary concerns has been to reduce surface state density and the focus is open on Metal Insulator Semiconductor (MIS) structures. However, the methodology and/or results could also be valid for laser-facet preparation.) Below, a few selected references are summarised, with more attention to plasma assisted nitridation schemes.
The article xe2x80x9cNitridation of GaAs using helicon-wave excited and inductively coupled nitrogen plasmaxe2x80x9d, Hara et al, J. Vac. Sci. Technol. B 16, 183 (1998) demonstrate nitridation of GaAs by special plasma treatment containing mixtures of nitrogen and argon, and/or, nitrogen and oxygen. However, pure nitrogen plasma is not commented upon. The authors show by X-ray photoelectron Spectroscopy (XPS) analysis that Gaxe2x80x94N bonds are formed and under certain conditions only small amounts of Ga and As sub-oxides were found. They show that nitridation suppresses oxide formation. The authors have investigated C-V characteristics of MIS devices using this procedure and found improvements. Further, photoluminescence yield is high for treated samples indicating lower surface/interface state densities. This work explicitly focuses on MIS aspects and there is no mention of the same procedure being applicable for pump lasers.
The article xe2x80x9cSurface cleaning and nitridation of compound semiconductors using gas-decomposition reaction in Cat-CVD methodxe2x80x9d, Izumi et al, Proc. Int. Vac. Congress, Aug. 31-Sep. 4, 1998, Burmingham, UK, describes the use of a gas-decomposition reaction involving Ammonia in a catalytic CVD (cat-CVD) system to for cleaning and nitriding GaAs surfaces. The authors use XPS to investigate the chemical bonding states near the surface. They claim disappearance of oxygen related peaks after their process. The proposal is that dissociation of ammonia results in hydrogen,which cleans the surface by removing the oxides, and in nitrogen, which forms Gaxe2x80x94N by a exchange reaction. That is, nitrogen efficiently replaces As This work mentions only MIS applications.
The articles xe2x80x9cNitridation of GaAs (110) using energetic N+ and N2+ ion beamsxe2x80x9d, L. A. DeLouise, J. Vac. Sci. Technol. A11, 609 (1993) and xe2x80x9cReactive N2+ ion bombardment of GaAs (110): A method for GaN thin film growthxe2x80x9d, J. Vac. Sci. Technol. A10, 1637 (1992) use XPS to analyse nitridation of GaAs (110) upon bombardment using nitrogen ion beams (500 eV to 3 KeV). It is demonstrated that lower surface densities are obtained with nitrogen compared to Ar and is attributed to the formation of stable predominantly Gaxe2x80x94N bonds. Again both these articles refer to MIS-like applications and the ion energies are relatively high.
The article xe2x80x9cNH3 plasma nitridation process of 100-GaAs surface observed by XPSxe2x80x9d, Masuda et al, J. J. Appl. Phys. Part 1, 34 1075 (1995) describes XPS studies of nitridation of GaAs using ammonia plasma show formation of Gaxe2x80x94Asxe2x80x94N layer. However, under certain conditions, the authors claim formation of only Gaxe2x80x94N layer due to desorption of As. They also report that the layer is oxidation resistant.
The article xe2x80x9cXPS investigation of GaAs nitridation mechanism with an ECR plasma sourcexe2x80x9d, Sauvage-Simkin et al, Phys. Stat. Solidi A176, 671 (1999) describes formation of beta-GaN in GaAs samples exposed to nitrogen ECR plasma from XPS studies. An amorphous layer formation is evidenced, which could favour nitrogen incorporation but should be controlled to stabilize Gaxe2x80x94N bonds.
The article xe2x80x9cIII-V surface plasma nitridation: A challenge for III-V nitride epigrowthxe2x80x9d, Losurdo et al, J. Vac. Sci. Technol. A17, 2194 (1999) describes the increased efficiency of nitridation in the presence of hydrogen. It is proposed that hydrogen enhances desorption of group V elements.
The article xe2x80x9cNanometer scale studies of nitride/arsenide heterostructures produced by nitrogen plasma exposure of GaAsxe2x80x9d, Goldman et al, J. Electronic Mat. 26, 1342 (1997) describes the use of a sophisticated tool, scanning tunnelling microscope (STM), to investigate plasma nitridation of GaAs. The authors find that the nitrided layer is not a continuous film, as also found in some other works reported above. Instead it is composed of defects (Asxe2x80x94N) and clusters (GaN with dilute As). These results show that defects that could be detrimental to device performance can also be formed. However, if appropriate nitridation conditions and possible annealing steps are used, the defects can be minimized.
The article xe2x80x9cSurface passivation of GaAs by ultra-thin cubic GaN layerxe2x80x9d, Anantathasaran et al, Appl. Surf. Sci. 159-160, 456 (2000) describes the use of a nitrogen plasma to form a thin cubic GaN layer and use XPS and RHEED to analyse the samples. All these processing were performed under Ultra High Vacuum (UHV) conditions. The PL measurements show an order of magnitude increase in intensity compared to as-grown samples indicating good passivation properties of the nitrided layer.
The main import from the literature is that nitridation of GaAs is possible using nitrogen plasma. Some articles above have also addressed nitridation by nitrogen-ion bombardment. Most reported works refer to MIS structures for motivation and no explicit reference to pump-laser facet passivation by nitridation is mentioned. Some reports also show that the formed nitrided layer is non-uniform and could require some additional processing steps such as annealing.
Two articles describe passivation of laser facets.
The article xe2x80x9cReliability improvement of 980 nm laser diodes with a new facet passivation processxe2x80x9d, Horie et al, IEEE Jour. of selected topics in quantum electronics 5, 832 (1999) demonstrates improved laser performance with a three step facet preparation. The laser bars are cleaved in air, thus increasing the yield. However, the facet preparation procedure involves three steps accomplished under vacuum conditions, making it somewhat complex. The procedure itself involves low-energy Ar-ion milling, followed by a-Si layer deposition and then finally an AlOx coating layer deposition. The problem here is that after Ar-milling, the surface cannot be exposed to ambient air. Nothing is mentioned about nitrogen milling.
The article xe2x80x9cA highly reliable GaInAsxe2x80x94GaInP 0.98 xcexcm window laserxe2x80x9d, Hashimoto et al, IEEE J of quantum electronics 36, 971 (2000) describes the use of implantation of nitrogen and subsequently RTA to cause atomic inter-diffusion near the active region at the facet. The basic mechanism is creation of defects by selective nitrogen implantation. Upon RTA, the defects assist in increased atomic inter-diffusion and cause the band-gap near the facet to increase (window laser). However, in this work the authors do not give details of implantation etc. The nitridation effect or rather the formation of dilute nitrogen containing alloy is not commented upon. Nevertheless, their procedure of nitrogen implantation and RTA, does show a band-gap increase of about 100 meV as seen from Photo-Luminescence (PL) measurements.
After cleaving a laser wafer into laser bars to provide a laser facet surface on each side of the bar, conventional surface cleaning methods such as as Ar-ion milling often degrades the crystal quality near the surface. Energetic Ar-ions impinging on the surface layer sputters away the native oxide layer formed when cleaving in surrounding air, but causes damage to the crystal itself. Typically, is after such a procedure, a near-surface damage-layer remains. The nature of this residual damage includes newly created defects (interstitials, vacancies etc.), stoichiometric damage in crystals which are composed of two or more constituent elements (e.g. GaAs, etc) resulting from preferential removal of some elements compared to the others, and a rough surface morphology. This defective layer, particularly in localized areas, can absorb photons causing progressive (accelerated) local heating leading to COD.
The Ar-ion milling process could also heat the crystal and cause out-diffusion of material followed by decomposition of the crystal.
An object of the invention is to provide a facet passivation procedure, which is simple, cost-effective and at the same time gives a high yield by improved reproducibility.
Another object of the invention is to provide a facet preparation procedure that satisfies both requirements, the minimizing of light absorption and surface recombination.
Still another object of the invention is to provide a facet preparation procedure that at least partially satisfies the above requirements. Such a simplified preparation procedure could be good enough for some application ranges.
The method according to the invention is to nitridise laser mirror facet of laser bars or laser chips during an etching process, such as milling with a gas comprising neutral nitrogen atoms or nitorgen ions in molecular and/or atomic form in a vacuum chamber. The facets of the laser bars were first cleaved in air, or some other ambient atmosphere. Introduction of a reactive gas like nitrogen in the etching process will certainly affect the crystal surface properties since it reacts with the crystal elements and creates a nitrided surface layer.
Thus, the etching process and the nitridisation may be performed with a plasma containing nitrogen ions, in molecular or atomic form, or neutral atomic nitrogen.
The essential concept behind this nitridisation is the formation of a nitride layer at the facet that
(a) prevents chemical contamination (for example oxidation),
(b) provides a higher band-gap surface layer, and
(c) possibly also reduces the surface/interface carrier recombination velocity.
Hydrogen gas during ion milling of the laser facets
(a) helps to clean the laser facet surface more effectively, especially the oxidized areas, since hydrogen is known to be effective in removing surface oxides, and
(b) aids in the removal of group V elements in a III-V crystal making formation of group III-nitrides more favourable.
The nitrided surface layer so formed on the facet surface during nitrogen ion milling could be reinforced, particularly to even out surface interruptions and pin holes, if any, by subsequent deposition of an additional nitride film which may contain an element from the groups 2b, 3a, 4a and 5a such as any of the following elements: Al, Si, Ga, C, Ga, Zn.
A contamination free surface is created either
(a) with, a surface nitrided layer so formed by nitrogen ion milling (with or without hydrogen),
(b) with a surface nitrided layer so formed by nitrogen ion milling (with or without hydrogen) and an additional over-layer of deposited nitride film., or
(c) with a mild nitrogen ion milling followed by nitridification by neutral atomic nitrogen.
Prior to mirror coating, the so-created contamination-free surface could be sealed by a passivation layer of such properties that non-radiative carrier recombination at the nitride-passivation layer-mirror coating interfaces is reduced to a minimum. Contrarily, direct deposition of mirror coating on the so-created contamination free surface may result in appreciable non-radiative carrier recombination via interface states at the nitride-mirror coating interface. (Passivation layers are often used in the prior art when the laser chips (bars) were cleaved in high vacuum and a specified passivation layer were usually deposited directly on the cleaved surface before the final mirror coating. Incidentally, the passivation layers are also used to change the reflectivity of the mirror coating. Passivation layers consisting of one or more of the following elements Zn, Se, S, Ga and N are typically reported.)
According to the invention a method of nitrogen ion-milling for laser facet preparation is promising:
(i) laser bars can be cleaved in air,
(ii) ion-milling in vacuum removes the native oxide layer, and
(iii) nitrogen (either ionic of atomic), if incorporated into the sample placed in the vacuum chamber, forms near-surface nitrided compounds which normally have band-gaps higher than their counter parts and can also prevent subsequent undesirable chemical contamination.